In the metallurgical industry there are different processes in which liquid metal is to be processed in one way or another. One example is the casting of metal, such as steel. In part of such a casting process, the liquid metal is supplied from a ladle to a tundish via a pouring nozzle which is generally referred to as a shroud. The metal flows from the tundish via another pouring nozzle to a casting mould or chill mould, in which the metal is cooled and transformed into solid form.
In smelting and heating processes there is a risk of slag being formed in the molten metal. In order to minimize the risk of slag being included in the final cast product, the casting plant may include a slag detection device. If the slag detection device detects slag in the molten metal an alarm is generated so that suitable measures may be taken.
In the prior art one type of slag detection device includes electromagnetic coils placed above a sliding gate in the ladle. This type of slag detection device works on the principle that for an electromagnetic field the penetration depth of metal is much less than the penetration depth of slag. Thus, a large portion of the electromagnetic field is allowed to pass through slag, similarly to air, and therefore it may be difficult for the device to distinguish slag from gas (such as air). In the bottom of the ladle, i.e. just above the sliding gate, this is not a significant problem since at the measuring area of the ladle it is generally filled with molten metal.
However, it would also be desirable to detect slag in the shroud between the ladle and the tundish, This would avoid the necessity of installing slag detecting electromagnetic sensors (coils) in every ladle used in a casting plant. Furthermore, sensors installed in a ladle are in such a position where there is a relatively high risk of mechanically damaging the sensors. Thus, it would be desirable to install electromagnetic slag detecting sensors in such manner that cut costs and minimizes the risk of mechanical damage to the sensors. Even though there exists slag detecting devices that comprise vibration meters or vibration meters combined with infrared measuring devices, there still lacks a satisfactory method of detecting slag in the shroud by means of electromagnetism. The reason for this is that the molten metal passing from the ladle through the shroud does not always completely fill the shroud. Consequently, the portion of the shroud which contains gas instead of metal may be mistaken for slag, resulting in the generation of a false alarm.